US7279120B2 - Doped cadmium tungstate scintillator with improved radiation hardness - Google Patents

Abstract

This invention provides novel cadmium tungstate scintillator materials that show improved radiation hardness. In particular, it was discovered that doping of cadmium tungstate (CdWO₄) with trivalent metal ions or monovalent metal ions is particularly effective in improving radiation hardness of the scintillator material.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of and priority to provisional application Ser. No. 60/500,917, filed on Sep. 4, 2003.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

[Not Applicable]

FIELD OF THE INVENTION

This invention provides new compositions and processing method for making doped cadmium tungstate scintillator materials that have a higher radiation hardness than that of undoped cadmium tungstate under UV, X-ray and other high-energy irradiation. The scintillator materials of this invention can be used in a variety of applications including, but not limited to X-ray detectors, e.g., for X-ray CT, digital panel imaging, screen intensifier etc. The scintillator materials of the invention can be used in bulk, sheet and film forms of ceramics, single crystals, glasses, and composites.

BACKGROUND OF THE INVENTION

A scintillator is a material, typically a crystal that responds to incident radiation by emitting light (e.g., a light pulse). Scintillators and compositions fabricated from scintillator materials are widely used in detectors for gamma-ray, X-rays, cosmic rays, and particles whose energy is of the order of 1 keV and greater. Often scintillators are used in X-ray imaging detectors for medical diagnostics, security inspection, industrial non-destructive evaluation (NDE), dosimetry, and high-energy physics. Using scintillator materials, it is possible to manufacture detectors in which the light emitted by the scintillator is coupled to a light-detection means and produces an electrical signal proportional to the amount of light received and to intensity of the incident radiation.

To acquire high resolution and quality picture, high dose radiation is required. Unfortunately, high dose radiation generally causes radiation damage to the scintillator material (i.e., the transmittance of scintillator crystal decreases with time), which inevitably leads to the loss of resolution and performance. One approach to solving this problem has been to lower the radiation dose. This approach requires better scintillators that provide higher light output to maintain the detector resolution and quality.

Recently, there has been an increasing demand for transparent, high atomic density, high speed and high light-output scintillator crystals and ceramic materials as detectors for computed X-ray tomography (CT) and other real time X-ray imaging systems. Many transparent ceramics like (Y,Gd)₂O₃:Eu³⁺, Gd₂O₂S:Pr,F,Ce, etc. have been developed for this purpose. Their slow response, however, and lack of single crystal form have limited their applications for X-ray explosive detection systems and X-ray panel displays.

Another approach to providing improved scintillator materials has been to improve the radiation hardness of the scintillator crystals so they can withstand high dose radiation. Scintillators typically used for x-ray explosive detection systems are mainly CsI and CdWO₄single crystals. Even though CsI provides a higher light output, its use has been problematic due to slow scan speeds associated with afterglow problems and low density for CsI,

CdWO₄crystals are more popular for X-ray explosive detection. CdWO₄possesses shorter afterglow times and demonstrates lower radiation damage than CsI. Unfortunately, the radiation damage of CdWO₄crystal also depends on the radiation dose. The higher the dose, the greater the radiation damage. For example, the radiation damage of CdWO4 crystal can reach as high as 40% when radiation dose is high enough.

Various approaches have been taken to reduce the radiation damage of tungstates. One approach has involved doping the scintillator material with other elements. Thus, for example, U.S. Patent Publication US 2003/0020044 A1, describes scintillator compositions formed from alkali and rare-earth tungstates, that have a general formula of AD(WO₄)_n. The composition CsY_0.25Gd_0.75W₂O₈doped with Ca, showed improved radiation tolerance.

Another approach to improving radiation tolerance of scintillators has involved annealing the scintillator crystal in a controlled atmosphere. For example, the damage mechanism of PbWO₄has been analyzed, and it was found the damage was caused by oxygen vacancies. By annealing the PbWO₄crystal in an oxygen atmosphere, the defects in the crystal structure decreased significantly and the resistance to radiation damage improved.

SUMMARY OF THE INVENTION

This invention pertains to the use of doping to improve the radiation resistance of CdWO₄. It was a discovery of the present invention that doping provides a better method of increasing radiation hardness (e.g., as compared to annealing) as the doping can stabilize the scintillator crystal structure in air, which is convenient for material preparation. Moreover the doping does not substantially interfere with the light output and other desirable properties of the material.

In particular, it was discovered that doping of cadmium tungstate (CdWO₄) with trivalent metal ions or monovalent metal ions is particularly effective in improving radiation hardness of the scintillator material.

Thus, in one embodiment this invention provides a scintillator composition comprising a cadmium tungstate doped with a monovalent metal ion and/or a trivalent metal ion, wherein said scintillator composition shows higher radiation tolerance that undoped CdWO₄. In certain embodiments, the doped cadmium tungstate is represented by the formula: Cd_(1−(x+y))A_xB_yWO₄, where A is a monovalent metal ion selected from the group consisting of Li, Na, K, Rb, Cs, Ag, Tl, and combinations thereof; B is a trivalent metal ion selected from the group consisting of Bi, Y, La, Ce, Pr, Nd, Gd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, and combinations thereof; X ranges from 0 to about 0.01; Y ranges from 0 to about 0.01; and at least one of X or Y is non-zero. In certain embodiments, x is about 0.0002, 0.0005, 0.0008, 0.001, 0.003, 0.005, or 0.01 and y is optionally zero or greater than zero (e.g., 0.0002, 0.0005, 0.0008, 0.001, 0.003, 0.005, or 0.01). In certain embodiments, y is about 0.0002, 0.0005, 0.0008, 0.001, 0.003, 0.005, or 0.01 and x is optionally zero or greater than zero (e.g., 0.0002, 0.0005, 0.0008, 0.001, 0.003, 0.005, or 0.01). In certain embodiments, B is Gd³⁺ or Bi³⁺ and y is zero or ranges from about 0.0002 to about 0.001. In certain embodiments, x and y are both greater than zero. In certain embodiments, K¹⁺ or Rb¹⁺ and B is Bi³⁺ and x and y are both greater than zero. In certain embodiments, the doped cadmium tungstate is selected from the group consisting of Cd_0.9995Na_0.0005WO₄, Cd_0.9995Gd_0.0005WO₄, and Cd_0.999(KBi)_0.001WO₄.

In various embodiments this invention also provides a detector element for an x-ray detector (e.g. an element of an x-ray CT scanner). The detector element typically comprises a scintillator as described herein and, optionally, a photodetector (e.g., photodiode, photomultiplier, film, optical guide, etc.).

Also provided is a method of making a scintillator composition. The method typically involves combining essentially equal amounts of CdO and WO₃and minor amounts of a dopant that comprises at least one oxygen-containing compound of a monovalent metal selected from the group consisting of Li, Na, K, Rb, Cs, Ag and TL, or/and at least one oxygen containing compound of a trivalent element selected from the group consisting of Bi, Y, La, Ce, Pr, Nd, Gd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm and Yb, where the dopant(s) comprise less than about 0.1 mole percent of the amount of cadmium; and firing the mixture at a temperature and for a time sufficient to convert the mixture to a solid solution of cadmium tungstate. Another method of making a scintillator composition comprises preparing a solution of a cadmium compound, a tungsten compound, and an amount of a monovalent metal ion selected from the group consisting of Li, Na, K, Rb, Cs, Ag, T, and/or a trivalent metal ion selected from the group consisting of Bi, Y, La, Ce, Pr, Nd, Gd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, where the monovalent ion and the trivalent ion when present are less than about 0.1 mole percent of the amount of cadmium; precipitating the compounds in a basic solution to obtain a mixture of oxygen-containing compounds; calcining the precipitate in an oxidizing atmosphere; and heating the precipitate at a temperature and for a time sufficient to convert the mixture to a solid solution of cadmium tungstate.

Also provided is an optical fiber comprising a scintillator composition of this invention where the optical fiber is optically coupled to the scintillator composition.

In still another embodiment this invention provides a method of producing an X-ray image. The method typically involves providing an x-ray detector comprising a scintillator composition as described herein, subjecting the scintillator composition to X-ray radiation; and detecting light emission from the scintillator composition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates the Czochralski method for synthesizing a scintillator material of this invention.

FIG. 2 schematically illustrates a portion of a CT machine comprising a scintillator material according to an embodiment of this invention.

FIG. 3 schematically illustrates an x-ray detector system according to an embodiment of this invention. The system can be used for a variety of purposes including, but not limited to medical diagnostics, airport scanning, explosive detection, and the like.

DETAILED DESCRIPTION

This invention provides novel improved cadmium tungstate scintillator materials that show increased resistance to radiation damage. In certain embodiments, the scintillator materials of this invention comprise a cadmium tungstate doped with a monovalent metal ion and/or a trivalent metal ion, such that the scintillator composition shows higher radiation tolerance that undoped CdWO₄.

In various embodiments the doped cadmium tungstate is represented by the formula:
Cd_(1−(x+y))A_xB_yWO₄  I.
where A is a monovalent metal ion selected from the group consisting of Li, Na, K, Rb, Cs, Ag, Tl, and combinations thereof; B is a trivalent metal ion selected from the group consisting of Bi, Y, La, Ce, Pr, Nd, Gd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, and combinations thereof; X typically ranges from 0 to about 0.01; Y typically ranges from 0 to about 0.01; and at least one of X or Y is non-zero. In certain embodiments, the cadmium tungstate is doped only with a monovalent metal ion (i.e., x is zero), in other embodiments, the cadmium tungstate is doped only with a trivalent metal ion (i.e., y is zero), and in certain embodiment, the cadmium tungstate is doped with both a monovalent metal ion and a trivalent metal ion. In certain embodiments, x and/or y are about or range up to about 0.008, 0.006, 0.004, 0.002, or 0.001. In certain embodiments, x and/or y are about, or range up to about 0.005.

The doped cadmium tungstate scintillator materials of this invention can be prepared according to a number of methods including, but not limited to the Bridgeman-Stockbarger method, conventional ceramic processes, a gelation/coprecipitation method, or a single crystal growth method such as the Czochralski method.

In a conventional ceramic process, essentially equal amounts of CdO and WO₃are combined, and to this combination is added the desired amount of the desired dopant(s), e.g., oxygen-containing compounds of at least one trivalent element selected from the group consisting of Bi, Y, La, Ce, Pr, Nd, Gd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm and Yb (e.g. Bi₂O₃, Y₂O₃, La₂O₃, Ce₂O₃, Pr₂O₃, Nd₂O₃, Gd₂O₃, Pm₂O₃, Sm₂O₃, Eu₂O₃, Tb₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, and the like), or/and at least one oxygen-containing compound of at least one monovalent metal selected from the group consisting of Li, Na, K, Rb, Cs, Ag and Tl (e.g., Li₂O, Na₂O, K₂O, Rb₂O, Cs₂O, Ag₂O, Tl₂O, and the like). In certain embodiments, the compounds with are wet-mixed together with ethanol alcohol to form a slurry. The slurry is dried, e.g., at 100° C. for 2 hours. The mixture is then fired at temperature and for a time sufficient to convert the mixture to a solid solution of cadmium tungstate (e.g. at a temperature from 900° C. to 1100° C., for several hours).

In a gelation/coprecipitation process, a solution comprising a compound of Cd, a compound of tungsten, and the desired dopant(s), e.g., at least one trivalent element selected from the group consisting of Bi, Y, La, Ce, Pr, Nd, Gd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm and Yb, or/and at least one monovalent metal selected from the group consisting of Li, Na, K, Rb, Cs, Ag and Tl is prepared with dilute HNO₃solvent, and then gelatinized by adding ammonia to the solution to obtain a gel of oxygen-containing compounds. The gel is dried, e.g., at 100° C. for 3 hours and then calcined, e.g., at 700° C. for 1 hour in an oxidizing atmosphere. The calcined sample is fired at a temperature ranging from 900° C. to 1200° C. and for a time sufficient to convert the mixture to a solid solution of cadmium tungstate.

The single crystal growth process for producing a scintillator composition include, but are not limited to, the Bridgeman-Stockbarger method and the Czochralski method. A schematic of the Czochralski crystal growth method is illustrated inFIG. 1.

Typically, seed crystal of CdWO₄is introduced into a saturated solution containing appropriate compounds and new crystalline material is allowed to grow and add to the seed crystal using Czochralski method.

The starting materials, typically comprising a cadmium compound, a tungsten compound and one or more of the dopant materials described herein are placed in acrucible534 and heated to form areactant melt530. The crucible is typically located in a housing, such as aquartz tube520, and heated, e.g. by r.f. orresistance heaters522. The melt temperature is determined by athermocouple532. A single crystal seed526 (e.g., a CdWO₄seed crystal) attached to aseed holder524 is lowered into the melt (the melt is the high temperature zone). As theseed526 is rotated about its axis and lifted from themelt530, a singlecrystal scintillator boule528 forms below the seed. The size of thecrystal boule528 increases as theseed526 is lifted further away from themelt530 toward the low temperature zone above theheaters522. The boule can then be sliced and polished into scintillator crystals.

The scintillator compositions of this invention can be used in many different applications. For example, the materials can be used as phosphors in lamps, in cathode ray tubes, in plasma display devices, or in a liquid crystal display. The material may also be used as scintillators in electromagnetic calorimeters, in gamma ray cameras, in computer tomography scanners, in x-ray detectors, in image intensifiers, in a lasers, and the like. These uses are meant to be merely illustrative and not exhaustive.

In certain preferred embodiments, the tungstate scintillator materials of this invention are fabricated to form sintered bodies that can then be incorporated into any of a number of devices. Methods of forming scintillator materials into sintered bodies are well known to those of skill in the art (see, e.g., U.S. Pat. Nos. 6,740,262, 5,013,696, 6,458,295, 6,384,417, and the like).

Thus, for example, in one approach, a powdered scintillator materials of this invention can be subjected to a cold isostatic pressing, e.g., under a pressure of about 200 MPa and then shaped. The resulting cold pressed material can be covered, e.g., with molybdenum foil, and charged into a capsule, e.g. a cylindrical capsule of tantalum for hot isostatic pressing. After the internal air is exhausted from the capsule the airtight capsule is completely sealed, e.g. by electron beam welding. Thereafter, the airtight vessel is subjected to a hot isostatic pressing (HIP), e.g., under the conditions of 1000° C. to 1500° C. and 150 MPa in an inert atmosphere, e.g., argon. After this is cooled, the resulting sintered ingot can be removed the metal container and, optionally, cut or machined into the desired shape. Finally the surface of the resulting material scintillator can be polished, e.g., with the abrasive powder of silicon carbide GC2000.

The scintillator materials of this invention can be incorporated into any of a number of devices such as gamma ray cameras/detectors, computer tomography scanners, x-ray detectors, image intensifiers, and the like using methods well known to those of skill in the art (see, e.g., U.S. Pat. Nos. 6,458,295, 6,384,417, 6,340,436, 5,558,815, and the like).

By way of illustration,FIG. 2 schematically illustrates a computer tomography (CT)scanning system210 utilizing a scintillator according to this invention. In certain embodiments, thisscanning system210 comprises acylindrical enclosure220 in which the patient or object to be scanned is placed. Asupport212 typically surrounds thecylinder220 and is configured for partial or full rotation about the cylinder's axis. Thesupport212 can be designed to revolve for one full revolution and then return or it can be designed for continuous rotation, depending on the system used to connect the electronics on the support to the rest of the system. Typically, the support includes anx-ray source214 that, in certain embodiments produces a fan-shaped x-ray beam that encompasses ascintillation detector system226 mounted on the support on the opposite side of thecylinder220. The pattern of the x-ray source is generally disposed in the plane defined by thex-ray source214 and thescintillation detector system226.

Thescintillation detector system224 is typically narrow or thin in the direction perpendicular to the plane of the x-ray fan beam. Eachcell226 of the scintillation detector system incorporates a solid transparent bar of a scintillator material of this invention and aphotodetector222, e.g. a diode, optically coupled to that scintillator bar.

The output from each photodetector is generally connected to an operational amplifier which is mounted on thesupport212. The output from each operational amplifier is connected either byindividual wires228 or by other electronics to themain control system218 for the computed tomography (CT) system. In the illustrated embodiment, power for the x-ray source and signals from the scintillation detector are carried to themain control system218 by acable216. The use of thecable216 may limit thesupport212 to a single full revolution before returning to its original position.

Alternatively, where continuous rotation of thesupport212 is desired, slip rings or optical or radio transmission may be used to connect the support electronics to themain control system218. In CT scanning systems of this type, the scintillator material is used to convert incident x-rays to luminescent light which is detected by the photodetector and thereby converted to an electrical signal as a means of converting the incident x-rays to electrical signals that can be processed for image extraction and other purposes.

FIG. 3 schematically illustrates a a fast response x-ray detector system incorporating one or more of the scintillator materials of this invention. In certain embodiments, the x-ray detector system includes an x-ray scintillator material of thisinvention306. Thescintillator material306 absorbsx-ray photons312 and emits scintillating radiation which is detected by thescintillating radiation photodetector308. Thescintillating radiation photodetector308 may be, for example, a photodiode, a photomultiplier, film, other solid state photo detectors, and the like. Preferably the response time ofphotodetector308 is faster than the primary decay time of the scintillating radiation in thescintillator material306 to take advantage of the short primary decay time of thescintillator material306.

Typically thescintillator material306 is optically coupled to thephotodetector308. Thescintillator material306 can be optically coupled to thephotodetector308 simply by positioning thescintillator material306 physically adjacent to thephotodetector308. In certain embodiments, thescintillator material306 can be optically coupled to thephotodetector308 by lens and/or mirrors and/or optical fiber(s) to focus the scintillating radiation upon thephotodetector308. In certain embodiments, thescintillator material306 can be optically coupled to thephotodetector308 by means of optical fibers that transmit the scintillating radiation from thescintillator material306 to the photodetector. Another alternative is to bond the scintillator material directly to the photodetector with an optical glue.

In certain embodiments, thephotodetector308 can be a single detector or it can comprise an array of detectors/detector elements. In the instance that thephotodetector308 is an array of detectors/detector elements, it is preferable that the scintillator material include some means of channeling the scintillating radiation from a region of the scintillator emitting scintillating radiation directly to the detector element directly underlying that region of the scintillator material. For instance, as with the embodiment of the invention ofFIG. 2, the scintillator material can be made up of separate solid transparent bars. These bars can be coated with a material to prevent the scintillation radiation from passing between the bars.

The electronic signal output of the photodetector can ultimately be connected to controlelectronics310. The electronic signal output may be amplified by a appropriate electronics, e.g., an operational amplifier (not shown), prior to being input to the control electronics. Thecontrol electronics310 typically collects the output signal(s) from the photodetector(s)308 corresponding to detection of scintillating radiation. The collected output signals can be further processed to produce an image corresponding to the detected x-rays as is well known in the art.

The x-ray detector system of this embodiment can also include anx-ray source302 that directsx-rays312 towards thescintillator material306. However, it is not necessary that the x-ray detector system include anx-ray source302. Instead, the x-ray source can be external to the x-ray detector system, or the x-rays may emanate from the object to be studied. For example, in x-ray detection applications such as astrophysics applications, x-rays may emanate from a body beyond the earth. If the x-ray detector system includes anx-ray source302, thecontrol electronics310 can optionally include a power source for supplying power to the x-ray source.

The x-ray detector system can also include a compartment orholder304 for holding an object to be studied. Thecompartment304 is typically located between thex-ray source302 and thescintillator material306. The dimensions of the compartment/holder304 will depend upon the particular application. For instance, an x-ray detector system for baggage inspection should be of a size to accommodate baggage, while an x-ray detector system for medical imaging should be of a size to accommodate a human being or animal.

In still another embodiment, this invention contemplates one or more light guides (optical fiber) optically coupled to a scintillator material of this invention. The signal produced by the scintillator material can be transmitted along the optical guide to one or more detector(s) as desired. In certain embodiments, a single optical guide is optically coupled to a scintillator material, while in other embodiments, multiple optical guides are optically coupled to a scintillator material. In the latter embodiment, it can be desirable for each optical guide to “sample” a discrete and independent region of the scintillator. In other embodiments, the optical guides may sample overlapping areas of the scintillator material.

The scintillator material can be optically coupled to the optical guide simply by positioning the scintillator material physically adjacent to the optical guide. In certain embodiments, the scintillator material can be optically coupled to the optical guide by lens and/or mirrors. Another alternative is to bond the scintillator material directly to the photodetector with an optical glue or to weld the scintillator material to the optical guide. In certain embodiments, the scintillator material can be fabricated as an integral component of the light guide. Methods of optically coupling the scintillator material to an optical guide (e.g., optical fiber) are well known to those of skill in the art (see, e.g., U.S. Pat. Nos. 6,504,156, 6,384,417, 5,558,815, 5,391,876, 5,386,797, 5,360,557, 5,318,722, and the like).

EXAMPLES

The following examples are offered to illustrate, but not to limit the claimed invention.

Example 1

Doped cadmium tungstates having the formulas Cd_0.9995Na_0.0005WO₄, Cd0.9995Gd_0.0005WO₄, and Cd_0.999(KBi)_0.001WO₄were fabricated were fabricated using conventional ceramic processing methods. Briefly, proper amounts of raw materials from CdO, WO₃, Na₂CO₃, K₂CO₃, Gd₂O₃, and Bi₂O₃were selected according to the desired formula (Cd_0.9995Na_0.0005WO₄, Cd0.9995Gd_0.0005WO₄, and Cd_0.999(KBi)_0.001WO₄). The compounds were wet-mixed together with ethanol alcohol to form a slurry. The slurry was dried at 100° C. for 2 hours. The mixture was then fired at 1100° C. for 3 hours in air.

The susceptibility to radiation damage of these materials was calculated based on the percentage change of scintillating peak intensity before and after X-ray exposure (145 KeV) for 5 hours. The distance between the samples and the x-ray source was 10 cm. After irradiation, the samples were immediately measured for luminescence properties immediately to eliminate any recovery effect.

Table 1 lists the light output before radiation and radiation damage of One CdWO₄single crystal sample (milled to powder), one commercial CdWO₄and the three different doped tungstates. Radiation damage was defined as the difference in light output before and after radiation divided by the light output before radiation.

Table 1. light output and radiation damage data of various CdWO₄powder samples.

	Sample	Peak value (no rad.)	Radiation damage

	Single Crystal	888	37.5%
	Polycrystalline Powder	342	30%
	Cd0.9995Na0.0005WO4	416	13.1%
	Cd0.9995Gd0.0005WO4	414	2.2%
	Cd0.999(KBi)0.001WO4	571	6.8%

The doped samples were sintered at 1000° C. for 3 hours.

Emission spectra of the powdered samples before and after irradiation were determined by exciting the sample with an X-ray source having peak energy of 8 keV produced from a copper anode at a power of 40 kV and 20 mA. Since all emission spectra of all of the samples had almost the same peak position and shape, we used peak height as the measure of light output.

Usually the radiation damage of scintillator crystals is measured from the change of transmittance of visible light before and after irradiation. Powder samples, however, are not transparent and thus this method is not feasible.

The high intensity radiation caused defects in crystal structure, and these defects trap the visible light excited by X-ray. Consequently the radiation damage lowers the light output of the scintillator materials. Therefore, the radiation damage can also be measured by the change of light output of the scintillator(s) (when exposed to a defined energy source) before and after irradiation. From the data in table 1, the radiation damage, up to 37.5% for the single crystal powder sample, was consistent with the data measured by transmission method, indicates that the data presented in the Table is a valid measure of radiation damage.

The light output of crystal samples was much higher than the other powder samples indicating that the radiation-induced defects in the in single crystals was less than the powder samples. The light output of all of the doped samples was greater than the commercial CdWO₄powder sample indicating that the doping reduced radiation damage.

As shown in Table 1, Gd³⁺ doping was the most effective. Co-doping of K¹⁺ and Bi3+ was better than single Na¹⁺ doping. Without being bound to a particular theory, it is believed the doping of trivalent ions can introduce excess oxygen ions to the crystal lattice, which later supplements the oxygen vacancy caused by radiation and thereby reduces the radiation damage. In conclusion, the doping described herein, especially trivalent ion doping of CdWO₄improves the radiation hardness of the tungstate scintillator while maintaining the scintillator light output and other desirable properties.

It is understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the appended claims. All publications, patents, and patent applications cited herein are hereby incorporated by reference in their entirety for all purposes.

Claims (20)

1. A sintered body comprising a scintillator composition comprising a cadmium tungstate doped with a monovalent metal ion and/or a trivalent metal ion, the scintillator composition being a component of the sintered body, wherein said scintillator composition shows higher radiation tolerance than undoped CdWO₄.

2. The sintered body ofclaim 1, wherein the doped cadmium tungstate is represented by the formula:

Cd_(1−(x+y))A_xB_yWO₄

where:

A is a monovalent metal ion selected from the group consisting of Li, Na, K, Rb, Cs, Ag, Tl, and combinations thereof;

B is a trivalent metal ion selected from the group consisting of Bi, Y, La, Ce, Pr, Nd, Gd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, and combinations thereof;

X ranges from 0 to about 0.01;

Y ranges from 0 to about 0.01; and

at least one of X or Y is non-zero.

3. A scintillator composition comprising a cadmium tungstate doped with a monovalent metal ion and/or a trivalent metal ion, wherein said scintillator composition shows higher radiation tolerance than undoped CdWO₄; and,

wherein the doped cadmium tungstate is represented by the formula:

Cd_(1−(x+y))A_xB_yWO₄

where:

A is a monovalent metal ion selected from the group consisting of: Li, Na, K, Rb, Cs, Ag, Tl, and combinations thereof;

B is a trivalent metal ion selected from the group consisting of: Bi, Y, La, Ce, Pr, Nd, Gd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, and combinations thereof;

wherein X is about 0.005; and,

Y ranges from 0 to about 0.01.

4. A scintillator composition comprising a cadmium tungstate doped with a monovalent metal ion and/or a trivalent metal ion, wherein said scintillator composition shows higher radiation tolerance than undoped CdWO₄; and,

wherein the doped cadmium tungstate is represented by the formula:

Cd_(1−(x+y))A_xB_yWO₄

where:

A is a monovalent metal ion selected from the group consisting of: Li, Na, K, Rb, Cs, Ag, Tl, and combinations thereof:

B is a trivalent metal ion selected from the group consisting of: Bi, Y, La, Ce, Pr, Nd, Gd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, and combinations thereof;

wherein X is about 0.001; and

Y ranges from 0 to about 0.01.

5. A scintillator composition comprising a cadmium tungstate doped with a monovalent metal ion and/or a trivalent metal ion, wherein said scintillator composition shows higher radiation tolerance than undoped CdWO₄; and,

wherein the doped cadmium tungstate is represented by the formula:

Cd_(1−(x+y))A_xB_yWO₄

where:

A is a monovalent metal ion selected from the group consisting of: Li, Na, K, Rb, Cs, Ag, Tl, and combinations thereof;

B is a trivalent metal ion selected from the group consisting of: Bi, Y, La, Ce, Pr, Nd, Gd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, and combinations thereof;

wherein X is about 0.001 or about 0.005; and,

wherein Y is 0.

6. The sintered body ofclaim 2, wherein Y is about 0.005.

7. The sintered body ofclaim 2, wherein Y is about 0.001.

8. The sintered body ofclaim 6, wherein X is 0.

9. The sintered body ofclaim 7, wherein X is 0.

10. The sintered body ofclaim 2, wherein X is zero, and Y is greater than zero.

11. The sintered body ofclaim 10, wherein B is Gd³⁺.

12. A scintillator composition comprising a cadmium tungstate doped with a monovalent metal ion and/or a trivalent metal ion, wherein said scintillator composition shows higher radiation tolerance than undoped CdWO₄; and,

wherein the doped cadmium tungstate is represented by the formula:

Cd_(1−(x+y))A_xB_yWO₄

where:

A is a monovalent metal ion selected from the group consisting of: Li, Na, K, Rb, Cs, Ag, Tl, and combinations thereof;

wherein X ranges from 0 to about 0.01; and,

B is Gd³⁺ and Y ranges from about 0.0002 to about 0.001.

13. A scintillator composition comprising a cadmium tungstate doped with a monovalent metal ion and/or a trivalent metal ion, wherein said scintillator composition shows higher radiation tolerance than undoped CdWO₄; and,

wherein the doped cadmium tungstate is represented by the formula:

Cd_(1−(x+y))A_xB_yWO₄

where:

A is a monovalent metal ion selected from the group consisting of: Li, Na, K, Rb, Cs, Ag, Tl, and combinations thereof;

B is a trivalent metal ion selected from the group consisting of: Bi, Y, La, Ce, Pr, Nd, Gd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, and combinations thereof;

wherein X and Y are both 0.01 or less; and,

wherein X and Y are both greater than zero.

14. The scintillator composition ofclaim 13, wherein A is K¹⁺ or Rb¹⁺ and B is Bi³⁺.

15. A scintillator composition comprising a cadmium tungstate doped with a monovalent metal ion and/or a trivalent metal ion, wherein said scintillator composition shows higher radiation tolerance than undoped CdWO₄; and,

wherein the doped cadmium tungstate is represented by the formula:

Cd_(1−(x+y))A_xB_yWO₄

where:

A is a monovalent metal ion selected from the group consisting of: Li, Na, K, Rb, Cs, Ag, Tl, and combinations thereof;

B is a trivalent metal ion selected from the group consisting of: Bi, Y, La, Ce, Pr, Nd, Gd, Pm, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, and combinations thereof;

wherein X ranges from greater than zero to about 0.01, and Y is zero.

16. A scintillator composition comprising a cadmium tungstate doped with a monovalent metal ion and/or a trivalent metal ion, wherein said scintillator composition shows higher radiation tolerance than undoped CdWO₄; and,

wherein the doped cadmium tungstate is represented by the formula:

Cd_(1−(x+y))A_xB_yWO₄

where:

A is a monovalent metal ion selected from the group consisting of: Na, K and combinations thereof;

B is a trivalent metal ion selected from the group consisting of: Bi, Gd, and combinations thereof;

wherein said doped cadmium tungstate is selected from the group consisting of Cd_0.9995Na_0.0005WO₄, Cd_0.9995Gd_0.0005WO₄, and Cd_0.999(KBi)_0.001WO₄.

17. A detector element of an X-ray CT scanner comprising a scintillator composition of any one ofclaims 1 through16.

18. An x-ray detector comprising:

a scintillator composition of any one ofclaims 1 through16; and

a photodetector disposed to detect light emitted by said scintillator composition.

19. The detector ofclaim 18, wherein said photodetector is selected from the group consisting of a photomultiplier, a photodiode, and photographic-film.

20. A method of producing an X-ray image, said method comprising:

providing an x-ray detector comprising a scintillator composition of any one ofclaims 1 through16;

subjecting said scintillator composition to X-ray radiation; and detecting light emission from said scintillator composition.

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